The objectives of the proposed research are as follows: (a) to correlate the morphological, physiological and physical-chemical data into an explanation of myofilament behavior in striated muscle by (1) further investigation by low-angle X-ray diffraction and, where applicable, electron microscopy of the effects of physical changes such as ionic strength, cation valancy, pH and hydration upon the interfilament spacing of living intact and skinned single muscle fibers and the extension of these studies by stroboscopic X-ray diffraction technique to include activated intact and skinned living single fibers, and (2) investigation of the tension generating capacity of the single intact and skinned muscle fibers as effected by these conditions as well as by controlled changes in the interfilament spacing; and (b) to determine the precise colloidal nature of the myofilament lattice of striated muscle, the understanding of which will define the mileau in which muscle contraction occurs. This must be determined and understood before any of the theories of muscle contraction which have been proposed by several investigators can be substantiated and thermodynamically proven.